1. Field of the Invention
The present invention relates to a thin-type wideband antenna used in a communication system that requires an ultra wideband and miniature antenna, such as a broadband Personal Area Network (PAN) using the Ultra Wide-Band (UWB) technique, for example.
2. Description of Related Art
To implement the broadband PAN using the UWB technique an ultra wideband and miniature antenna are utilized. The so-called patch antenna (thin-type antenna) answers the requirement especially for the thin-type. The patch antenna is constructed of an insulating substance interposed between a radiation conductor and a reference conductor which are in facing relationship with respect to each other.
The shape of the radiation conductor is not especially restricted, however in general, a rectangular shape or circular is used. Generally, the thickness of the insulating substance interposed between the radiation conductor and the reference conductor is selected to less than 1/10 of the wavelength of the radio frequency. Accordingly, it can be made extremely thin.
The patch antenna can be manufactured comparably easily through the etching processing of an insulating substrate with copper layers spread on both the sides thereof. That is, the patch antenna is comparably easy of manufacturing, and it has an advantage of easiness in integration with a circuit board.
However, the patch antenna has a sharp operational bandwidth. Therefore, it is not suitable for the PAN system that requires a wider operational bandwidth. Suppose a patch antenna formed by using an insulating substance having a relative dielectric constant εr=4, conductivity σ=0.003 [/Ωm], and thickness t=2 mm as an interposition, and facing a square reference conductor whose length of the side is 68 mm and a square radiation conductor whose length of the side is 15 mm so that the centers of the two coincide. In this patch antenna, the center of the reference conductor and the center of the radiation conductor are connected with a short-circuiting pin, and a feeding point is provided at a position 3 mm remote from the short-circuiting pin. The simulation result of this patch antenna is as follows:
FIG. 19A is a Smith chart illustrating the impedance characteristic of the patch antenna having the above parameters, and FIG. 19B illustrates the VSWR characteristic of the same. FIG. 20A illustrates a radiation pattern characteristic obtained by radiating a signal of the frequency f=3.5 GHz, FIG. 20B illustrates a radiation pattern characteristic obtained by radiating a signal of the frequency f=4 GHz, and FIG. 20C illustrates a radiation pattern characteristic obtained by radiating a signal of the frequency f=4.5 GHz.
As understood from FIG. 19, when the operational bandwidth is regarded as a bandwidth, in which the VSWR is less than 2, only a relative bandwidth of about 3% can be obtained. As understood from the comparison of FIG. 20A, FIG. 20B, and FIG. 20C, the case using the signal of the frequency 4 GHz achieved a satisfactory gain, however both the case using the signal of 3.5 GHz and the case using the signal of 4.5 GHz could not achieve a sufficient gain. Thus, there has been a desire for a thin-type wideband antenna with a lowered standing wave ratio that follows the advantage of easiness in production and easiness in integration with a circuit board, and so forth that the patch antenna has, and which is applicable to a communication system that requires a wider bandwidth, such as the PAN system.